Fundamental aspects of the biology of human immunodeficiency virus (HIV) infection, such as the anatomical location of ongoing viral replication during potent antiretroviral therapy, remain undefined. Evaluating pathophysiological questions during HIV disease is hampered by techniques that are too invasive to perform on human subjects. Investigations must therefore rely on laboratory based ex vivo experiments, autopsy specimens, or animal models that are plagued with potential artifacts that obscure extrapolation of results to natural infection. Positron emission tomography (PET) technology, however, may provide a powerful and safe technique to study the pathogenesis of HIV disease in vivo. Specifically, PET may be able to non-invasively detect HIV infection and localize tissues of active viral replication. We designed a pilot study to establish our ability to employ PET imaging to detect HIV infection in healthy HIV-infected subjects and to evaluate the impact of disease stage and HIV therapy on PET scan patterns. Additionally, in the second phase of the study, we are attempting to use PET technology to identify the site of ongoing viral replication during effective HIV treatment. Preliminary results indicate that PET scans of healthy HIV-infected patients clearly differ from images of HIV-uninfected controls. In particular, scans of four patients with high viral burdens (> 50,000 copies/ml) and relatively preserved immune systems (CD4+ T cell counts > 200 cells/mm3) demonstrated abnormal PET signal in multiple lymph node chains, a finding not seen in uninfected controls. These patterns may reflect areas of active viral replication. Of note, PET scans of two healthy individuals with advanced HIV disease (CD4+ T cells < 100 cell/ mm3 and plasma viremia > 50,000 copies/ml) were similar to images of patients with early disease. These findings do not support the hypothesis that deep lymph tissues (e.g. in the gut) play a more important role than peripheral lymph node chains in the maintenance of viremia in late-stage disease. In the second phase of our project, we are attempting to use PET technology to identify the site of ongoing viral replication during HAART. To date, three of the planned ten subjects have undergone serial PET scanning in the setting of HAART withdrawal. Although these patients have generated intriguing imaging results, it is too early in the investigation to draw any solid conclusion on the utility of PET to detect areas of ongoing viral replication during HAART.